Water purification technology is rapidly becoming an essential aspect of modern life as conventional water resources become increasingly scarce, municipal distribution systems for potable water deteriorate with age, and increased water usage depletes wells and reservoirs, causing saline water contamination. Additionally, further contamination of water sources is occurring from a variety of activities, which include, for example, intensive agriculture, gasoline additives, and heavy toxic metals. These issues are leading to increasing and objectionable levels of germs, bacteria, salts, MTBE, chlorates, perchlorates, arsenic, mercury, and even the chemicals used to disinfect potable water, in the water system.
Furthermore, even though almost three fourths of the earth is covered by oceans, only some 3% of this water exists as fresh water resources, and these resources are becoming increasingly scarce as a result of population growth and global warming. Approximately 69% of all fresh water is contained in ice caps and glaciers; with increased global melting, this fresh water becomes unrecoverable, so less than 1% is actually available, with the majority (over 90%) being ground water in aquifers that are being progressively contaminated by human activities and saline incursions. Thus, there is an urgent need for technology that can turn saline water, including seawater and brine, into fresh water, while removing a broad range of contaminants.
Conventional desalination and water treatment technologies, including reverse osmosis (RO) filtration and thermal distillation systems, such as multiple-effect distillation (MED), multiple-stage flash distillation (MSF), and vapor compression distillation (VC), are rarely able to handle the diverse range of water contaminants found in saline environments. Additionally, even though they are commercially available, they often require multiple treatment stages or some combination of various technologies to achieve acceptable water quality. RO systems suffer from the requirement of high-water pressures as the saline content increases, rendering them expensive in commercial desalination, and they commonly waste more than 40% of the incoming feed water, making them progressively less attractive when water is scarce. Moreover, RO installations produce copious volumes of waste brine that are typically discarded into the sea, resulting in high saline concentrations that are deadly to fish and shellfish. Less conventional technologies, such as ultraviolet (UV) light irradiation or ozone treatment, can be effective against viruses and bacteria but seldom remove other contaminants, such as dissolved gases, salts, hydrocarbons, and insoluble solids. Additionally, while most distillation technologies may be superior at removing a subset of contaminants, they rarely can handle all types of contaminants.
Because commercial desalination plants are normally complex engineering projects that require one to three years of construction, they are typically capital intensive and difficult to move from one place to another. Their complexity and reliance on multiple technologies also make them prone to high maintenance costs. Because RO plants are designed to operate continuously under steady pressure and flow conditions, large pressure fluctuations or power interruptions can damage the membranes, which are expensive to replace; the incoming feed water therefore requires extensive pre-treatment to prevent fouling of the RO membrane.
Thermal distillation systems, such as those described by LeGolf et al. (U.S. Pat. No. 6,635,150 B1) include MED systems, which rely on multiple evaporation and condensation steps that operate under vacuum in order to effect evaporation at temperatures lower than the normal boiling point of water. Such technologies are commercially used for desalination in various countries, but they all operate according to different physico-chemical principles. For example, MED, MSF, and VC systems all require vacuum, which means that the product water cannot be sterilized because evaporation occurs at temperatures lower than those needed for sterilization; also, vacuum systems tend to leak and require mechanical reinforcement. In addition, heat transfer and heat recovery in MED, MSF, and VC systems involve heat exchange across membranes or thin metal surfaces, but heat exchangers are prone to fouling and scale formation and require frequent maintenance.
More recently, Thiers (PCT Application No.: US2009/57277, entitled Large Scale Water Purification and Desalination, filed Sep. 17, 2009, and PCT Application No.: US2010/030759, entitled Method and System for Reduction of Scaling in Purification of Aqueous Solutions, filed Apr. 12, 2010) has described a method of pre-treatment that removes scale-forming constituents from a water stream and large scale embodiments for a desalination system. However, the earlier pre-treatment system described by Thiers relies on a final thermal treatment that involves heating to 120° C. for several minutes of residence time, which, while technically effective, represents a significant energy consumption. There is a need for a pre-treatment method that minimizes energy consumption while still removing scale-forming constituents from an aqueous stream. In addition, the embodiments described by Thiers for a large-scale desalination and water treatment fail to address transient phenomena encountered during start-up and shut down operations and do not properly ensure the maintenance of a stable hydraulic head between different boiling stages. There is a need for industrial configurations that are stable during start-up and shut down procedures, in addition to being stable during normal operation.
There is a need for inexpensive and effective pre-treatment methods that eliminate scale-forming compounds. There is a further need for industrial desalination and water treatment systems that are continuous and largely self-cleaning, that resist corrosion and scaling, that are modular and compact, that recover a major fraction of the input water while producing a highly concentrated waste brine that crystallizes into a solid salt cake, and that are relatively inexpensive and low-maintenance.